In order to improve the success rate of root-canal treatment in general dental practice, the referral of difficult cases to dentists with advanced knowledge and training in endodontics should be made possible for the benefit of patients (de Cleen et al. 1993, Saunders et al. 1997, de Moor et al. 2000). In order to be able to refer patients with complex endodontic problems, at least two requirements should be met: (i) sufficient endodontists must be available to handle the demand for specialist endodontic care; and (ii) general practitioners must be able to judge the difficulty of treatment required.